Provide a code point iterator to the String class to simplify iteration.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 07cb5bf6810a4918fbf3a6ffd74f419c27709944..793b913ed180877d8eb8c6b763048b51ea1d8814 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -3708,6 +3708,19 @@ class String : public Instance {
   static const intptr_t kSizeofRawString = sizeof(RawObject) + (2 * kWordSize);
   static const intptr_t kMaxElements = kSmiMax / kTwoByteChar;
 
+  class CodePointIterator : public ValueObject {
+   public:
+    explicit CodePointIterator(const String& str) : str_(str), index_(-1) {}
+
+    int32_t Current();
+
+    bool Next();
+
+   private:
+    const String& str_;
+    intptr_t index_;
+  };
+
   intptr_t Length() const { return Smi::Value(raw_ptr()->length_); }
   static intptr_t length_offset() { return OFFSET_OF(RawString, length_); }